Various operations in industry, agriculture, and elsewhere must be caused to occur automatically in an established sequence. One example is in irrigation where irrigation valves must be opened and closed in a known sequence. This sequence must be repeated periodically, such as every other day at prescribed times during the day.
Automatic sprinkler controllers which perform the above-described control functions are known and available commercially. One such automatic controller slowly rotates a selector at constant speed over a series of contacts. Engagement of the selector and one of the contacts completes a circuit to one or more of the irrigation valves to open such valve. The valves remain open for as long as the selector engages the associated contact.
This type of controller is reliable; however, it possesses several disadvantages. First, because of the construction of the controller, there are gaps between adjacent contacts. When the selector is in one of these gaps between contacts, no circuit is completed to an irrigation valve, and consequently, no irrigating takes place. This gap or deadband is undesirable because it is normally desirable to complete the irrigation process as rapidly as possible and without any interruption.
This prior art controller does allow the length of time for irrigation at each station to be varied. This is accomplished by using a wedge-shaped contact which can be moved progressively into, or out of, the path of movement of the selector to thereby vary the length of time of engagement between the selector and the contact. A station can be skipped entirely by removing the contact from the path of the selector. However, to the extent that the watering time at a station is diminished, the deadband increases. In other words, if the watering time at a station is decreased by ten minutes, the deadband, or nonwatering time, for the entire irrigation system is increased by ten minutes.
In an effort to overcome these disadvantages, two motors have been utilized on the controller. One motor drives the selector at a relatively slow speed over the contacts and the second motor drives the selector at a higher speed through the gaps. This controller is more expensive because of the additional motor and the additional controls for the motor. In addition, this controller only reduces the gaps, but does not eliminate them.
One form of controller which apparently has solved the deadband problem is substantially entirely electronic. However, this controller is very expensive.